Semiconductors are manufactured to close tolerances on very clean surfaces. These surfaces, typically silicon or gallium arsenide, are in the form of wafers that are manufactured in standard sizes of 2, 3, 4, 5, 6, 8, and 10 inches in diameter. Industry standards dictate that these pure crystalline wafers be of a certain thickness, resistivity, etc. These wafers are handled from the backside only by the means of a vacuum wand. They are kept sealed until they are used and can only be opened in a clean room environment to ensure that they are not contaminated by particles and other foreign materials that affect yield levels.
Many companies use optical surface inspection machines that use a laser or other technique to scan these wafers for particle contamination that cannot be seen without the use of a microscope or a SEM (Scanning Electron Microscope). Wafers are scanned for certain particle count levels and, if acceptable, they are used to build semiconductors on. If they are dirty, the wafers are cleaned and reinspected for contamination. If the wafers cannot be cleaned, they are scrapped because the cost of further processing exceeds the value of the wafer yield(s).
To guarantee accurate count estimates, optical surface inspection systems must be calibrated with standard particle size artifacts, such as polystyrene latex (PSL) spheres. A surface particle sizing standard is created when monosized PSL spheres that are traceable to the National Institute of Science and Technology are deposited onto the substrate of interest, e.g., bare silicon, silicon with a thin film coating, a patterned silicon wafer, a photomask or a disk.
In these systems, the sizing standard or wafer is scanned by a surface inspection instrument to determine the optical light scattering of the spheres deposited on the specific substrate. The amount of light scattered is then related to the size of the sphere deposited. When many substrates, such as wafers, are deposited with many different sphere sizes, one may calibrate the inspection machine across its entire dynamic range for that specific substrate type. The dynamic range of an instrument is the range over which particle sizes can be resolved from each other for a given substrate type. The ability to measure and resolve particles on substrates is important in the manufacture of many substrate types, but particularly in the production of semiconductors. The geometry of a given integrated circuit design dictates the sensitivity of a device to particles during manufacturing. The more integrated and dense the geometry, the smaller the particle size that can cause device failure, or functional yield loss. A 10:1 design rule is often used to identify particle sizes which can cause device failure, i.e., particles 1/10th the size of the geometry.
Most integrated circuit manufacturers produce several different device geometries and integration levels, thus each has different contamination limits. Surface inspection systems must therefore be set up with substrate particle sizing standards for each device type to provide relevant process control information to decision-makers and to provide an adequate return-on-investment in the inspection system itself.
One technique to apply monosized PSL sphere artifacts on the wafer (or other) surface incorporates chemical vapor deposition (CVD) principles. Typically, in such a deposition scheme, a colloidal suspension of PSL spheres of a known size distribution is made from concentrated spheres and surfactants diluted with ultra-pure deionized water. This suspension is atomized with an atomizer to produce an aerosol. The atomizer is typically designed for sizes of spheres from 0.005 micron to 1.0 micron in diameter. Such atomizers are designed to produce droplets of water that ideally contain one sphere per drop. Next, the atomized droplets leave the atomizer and flow through a line or tube to a glass expansion air dryer comprised of a connected series of four glass bulbs. The output of the glass dryer is a dry air/sphere mixture that is either reprocessed through the dryer if any visible moisture is evident or continues on to an optional electrostatic classifier. The classifier uses electrostatic forces to separate all incoming particles based on the electric mobility of each individual particle.
The dry air/sphere mixture then enters the deposition chamber and is dispersed by a nozzle at the top of the deposition chamber. The PSL spheres are pushed by the constant output from the nozzle toward the wafers which are positioned on deposition plates at the bottom of the chamber.
Existing deposition systems including systems for the deposition of these latex spheres have a number of drawbacks making them unsuited to commercial or research applications. For example, the available deposition plates for latex sphere deposition do not allow for batch processing, i.e., only one wafer fits in the deposition chamber at a time. Moreover, when only one half of the wafer is to be processed, the space between the wafer and the mask covering the half of the wafer is so large that some of the deposition material (latex spheres) can rebound off the uncovered portion of the wafer and land in the covered portion, thereby destroying the value of the wafer. Additionally, the devices covering one half the wafer are not adjustable for wafers already having layers of material on them from previous processing.
Yet another problem with existing deposition plates is the lack of means on the deposition plate to remove the wafer with a vacuum wand
Another drawback of existing deposition chambers relates to the inability of existing chambers to ensure uniform distribution of the particles being deposited on the wafer. One attempt to overcome this problem is seen in U.S. Pat. No. 4,989,541 to Mikoshiba et al. Mikoshiba et al incorporates a separate nozzle for a "control gas" which encircles the material gas flow and forcibly reorients laterally expanding portions of the material gas flow. The use of such a control nozzle and gas is both expensive and difficult. Moreover, it does not address problems faced when the ionized particles attach to the side walls of the mixing chamber.
Still another drawback relates to the time needed between the processing of each wafer in order to remove previous particles from the chamber. A lengthy amount of time is required between when the particles exit the nozzle and when they reach the base of the deposition chamber.